1. Field of the Invention
The present invention relates to a toner for use in electrophotography. In addition, the present invention also relates to an image forming method, an image forming apparatus, and a process cartridge using the toner.
2. Discussion of the Background
As electrophotographic technology is developed, image forming apparatuses using electrophotography such as laser printers, copiers, and multi function peripherals (i.e., MFP) are widely used in offices.
Recently, demands for low-price, easy-maintenance, and compact apparatus, which does not give an oppressive feeling even if being set beside a desk, have increased. In addition, demands for image forming apparatus which easily produces appealing color images have increased.
A typical image forming apparatus using electrophotography uses a toner including at least a binder resin and a colorant, and an image is typically formed as follows:                (1) An image bearing member (e.g., a photoreceptor drum, a photoreceptor belt) is evenly charged with a charging means (e.g., a charging roller, a charging brush) (i.e., charging process);        (2) The image bearing member is irradiated with a light source (e.g., a laser, a light-emitting diode) so as to form an electrostatic latent image thereon (i.e., irradiating process);        (3) A toner bearing member (e.g., a developing roller, a developing sleeve) approaches or contacts the image bearing member so as to develop the electrostatic latent image with a toner to form a toner image (i.e., developing process);        (4) The toner image formed on the image bearing member is optionally transferred onto a transfer member (e.g., an intermediate transfer roller, an intermediate transfer belt) (i.e., transfer process);        (5) The toner image formed on the image bearing member or the transfer member is transferred onto a recording member (e.g., a paper, an overhead projection (OHP) sheet) (i.e., transfer process); and        (6) The toner image formed on the recording member is melted upon application of pressure and heat from a fixing thereon (e.g., a fixing roller, a fixing belt) so as to be fixed thereon.        
In particular, a full-color image can be formed as follows.
(1) The above charging, irradiating, developing, and transfer processes are repeatedly performed using a least 2 toner bearing members, each of which includes a different-color toner, to form a full-color toner image on the transfer member or the recording member, and then the full-color toner image is subjected to the succeeding processes (i.e., four-cycle method).
(2) The above charging, irradiating, developing processes are simultaneously performed using at least 2 sets of an image bearing member and a toner bearing member, each of which includes a different-color toner, to form a single-color toner image on each of the image bearing members. Plural single-color toner images are transferred onto the transfer member or the recording member to form a full-color toner image thereon, and then the full-color toner image is subjected to the succeeding processes (i.e., tandem method).
Dry developing methods used for electrophotography or electrostatic recording are classified into two-component developing methods using a two-component developer consisting of a toner and a carrier, and one-component developing methods using a one-component developer consisting of a toner.
In general, the two-component developing methods can stably produce high quality images. However, the carrier tends to deteriorate and the mixing ratio between the toner and the carrier tends to vary with long-term use, and therefore high quality images cannot be produced or a long period of time. There are other drawbacks that the image forming apparatus using the two-component developing method is not easy to perform maintenance, and is difficult to be smaller in size.
Therefore, one-component developing methods, which have no drawback mentioned above, receive attention recently.
In a typical one-component developing method, at least one toner transport member transports a toner (i.e., a one-component developer) to visualize an electrostatic latent image formed on an image bearing member. It is considered that a toner layer formed on the toner transport member should be as thin as possible.
This is because the one-component developer (i.e., a toner having high electric resistance) needs to be charged by a developing device. When the toner layer is too thick, only toner particles present at the surface of the toner layer are charged, i.e. all of the toner particles cannot be evenly charged.
For the above reason, various methods for controlling toner layer thickness have been proposed.
For example, a control blade is proposed. The control blade is arranged facing the toner transport member, and presses the toner layer to the toner transport member to control the toner layer thickness.
A control roller can also be used instead of the control blade.
In attempting to satisfy the demands for low-price, easy-maintenance, and compact apparatus, oilless fixing processes, in which no fixing oil is used or less amount of fixing oil is used in a fixing process, have been proposed.
When a toner is melted in a typical fixing process, the toner tends to adhere to a fixing member, and therefore the fixing member cannot separate from the recording member having the toner thereon. As a result, the recording member cannot be discharged from the apparatus, resulting in occurrence of a paper jam. In attempting to prevent the occurrence of the paper jam, the following technique is proposed in which an oil applicator for applying an oil such as a silicone oil to the fixing member is arranged so as to separate the recording member therefrom. However, this technique has a drawback that the oil applicator takes up a large space in the apparatus, and therefore the apparatus becomes larger in size. Another drawback is that users using the apparatus have the trouble of supplying new oil when the oil is consumed. En a tempting to solve the problems of the oil applicator, a technique in which a wax is added to a toner is proposed. When such a toner is used in a fixing process, the wax exudes from the toner to the interface between the toner and the fixing member, and decrease adherence therebetween. As a result, the fixing member and the recording member can well separate from each other (this property hereinafter referred to as separativeness). A fixing process using a toner including a wax is called an oilless fixing process.
In order to impart good separativeness to a toner, a wax is preferably dispersed toner Generally speaking, waxes have low polarity, and therefore waxes are incompatible with toner binder resins which typically have functional groups Therefore, the wax particles tend to expose at the surface of the toner particles or release therefrom, when the toner is manufactured. The wax particles released from the toner particles tend to contaminate image forming members and deteriorate fluidity of the toner. When such a toner is used for a one-component developing method, in which a toner present on a toner bearing member is evenly charged while forming an even toner layer by rotating itself under a pressure applied by a toner layer control member (e.g., a control blade), the wax and fluidizers tend to firmly stick to the toner layer forming member. As a result, the toner layer is distributed, and therefore the quality of the resultant toner image largely deteriorates.
In attempting to solve the above problems, various techniques have been proposed.
For example, a technique in which the amount of a wax added to a toner is decreased and the elasticity of a binder resin is increased is proposed. In this case, the toner is hardly melted, and therefore fixability of the toner, and glossiness and image quality of the resultant images deteriorate.
In attempting to solve this problem without deteriorating developability and separativeness of the resultant toner, a technique in which a resin internally containing a wax is used as a binder resin of a toner is proposed.
For example, published unexamined Japanese Patent Application No. (hereinafter referred to as JP-A) 2001-117261 discloses a toner including a specific wax. Since wax has too high a melting point, the wax takes a long time to melt at low temperatures, and therefore the wax insufficiently exudes from the toner. Such a toner has insufficient separativeness. JP-A-2002-148844 discloses a toner having a high thermal property, and JP-A 2003-270859 discloses a toner including a large amount of a resin having high melt-viscosity. These toners produce low-glossiness images having poor image quality and poor fixing strength. JP-A 2003-255587 discloses a toner including a resin internally containing a wax. Since the resin internally containing a wax has a low softening point, the wax tends to expose at the surface of the toner and exert a bad influence on the image forming processes.
JP-A 06-324525 discloses an image forming method using a toner in which a large amount of an external additive is added thereto to improve transferability thereof. When the amount of the external additive is too large, the external additive inhibits fusing of the toner particles when the toner is fixed upon application of heat. In addition, excessive released external additive particles cause offset problem and deterioration of the fixing strength.
As mentioned above, a toner containing a wax has various drawbacks when used for a non-magnetic one-component developing method in which the toner is contact-charged by a contact control member such as a doctor blade and a doctor roller.
When such a wax-containing toner is used for a non-magnetic one-component developing method, the wax is required to exude from the toner in the fixing process so that the toner has good separativeness. On the other hand, the wax is not required to firmly adhere to the contact control member in the charging process. However, these two requirements are difficult to satisfy at the same time.
In order to satisfy both of the requirements at the same time, the wax is preferably added to a binder resin when the binder resin is synthesized. In other words, the wax is preferably mixed with monomers of the binder resin before the monomers are subjected to a polymerization. (This method is hereinafter referred to as internal addition method.)
In this case, the wax is incompatible with the binder resin, and it is easier to control the condition of the wax domains present in the binder resin compared with the case where a wax is mixed with a synthesized resin.
When a wax is added to a binder resin when the binder resin is synthesized, a wax having a functional group in a molecular chain thereof such as a carnauba wax having an ester bond formed from a higher alcohol and a higher fatty acid and an amide wax having an amide bond are not preferably used. This is because there is a possibility that such a wax reacts with the monomers of the binder resin. In this case, the wax cannot exert its effect and cannot impart good separativeness to the resultant toner.
For this reason, a hydrocarbon wax having no functional group is preferably used for the internal addition method.
The hydrocarbon wax can impart good separativeness to the resultant toner. However, the polarity thereof is largely different from that of the binder resin (compared with a wax having a functional group), and therefore the wax and the binder resin are incompatible and not sufficiently mixed with each other. In other words, the wax domains are present in the binder resin and form a sea-island structure.
In this case, the toner is required to have good mixing balance between the wax and the resin so that the toner properties are highly balanced.